PROJECT SUMMARY/ABSTRACT Binge eating disorder (BED) is the most prevalent eating disorder and is associated with substantial psychiatric and medical comorbidity. Although obesity is not part of the diagnostic criteria for BED, more than 65% of individuals with BED are obese, and more than a quarter of patients seeking treatment for obesity present with BED. To date, although there exist several effective treatments for reducing binge eating, facilitating clinically significant weight loss in patients with BED remains a challenge. We believe the failure of existing treatment approaches is related to 1) a lack of emphasize on altering the calorie balance (through both diet and physical activity) without encouraging strict dietary restraint that could promote an eventual re- occurrence of binge eating pathology, 2) limited strategies designed to promote long-term adherence to dietary and physical activity goals, and 3) the failure of both standard behavioral weight loss treatments (SBT) and cognitive behavioral therapy (CBT) to address key maintenance factors for BED (e.g. elevated food reward sensitivity, overvaluation of weight and shape, negative affect and distress intolerance). Acceptance-based behavioral treatment (ABBT) can provide individuals with the psychological tools necessary to improve negative affect, distress intolerance, and overvaluation of weight and shape and support long-term adherence to the dietary and physical activity recommendations designed to produce weight loss despite elevated food reward sensitivity. Our existing pilot data support the ability of ABBTs to produce superior weight loss outcomes both for obese patients at large and for obese patients with vulnerabilities similar to those observed in BED. ABBTs have also been shown to effectively reduce binge eating episodes in BED. Our pilot data suggests that ABBT could produce superior weight loss outcomes for patients with BED compared to SBT. The primary goal of the proposed project is to evaluate the efficacy of ABBT in relation to SBT for facilitating weight loss and weight loss maintenance in patients with BED. A secondary goal is to test hypothesized mechanisms of action of the two treatments, both during active intervention and during the post-treatment weight loss maintenance phase. Lastly, we aim to evaluate moderation hypotheses stating that the superiority of ABBT will be especially pronounced for those with higher food reward sensitivity, overvaluation of weight and shape, negative affect, and distress intolerance. Our aims work towards longer-range goals of identifying more effective methods for improving weight loss outcomes, using evidence to maximize the effective components of interventions, and matching patients to treatment type. Accordingly, we will randomly assign 130 overweight and obese patients with BED to 25 sessions of ABBT or SBT. All participants will be followed until one year post-treatment. Lab-based behavioral assessments, clinician guided interventions, and self- report measures will be used to provide a multi-method assessment of hypothesized moderating and mediating pathways, and how these associations are affected by treatment.